Using Ground-based Magnetics, VLF, and DC Resistivity to Examine Faulting at the San Andreas Oasis, Dos Palmas Preserve

Stacey R. Petrashek, Nathan W. Pulver, Raul Contreras, Drew Faherty, & Jascha Polet

Published August 14, 2019, SCEC Contribution #9569, 2019 SCEC Annual Meeting Poster #137

The San Andreas Oasis is located northeast of the Salton Sea near the convergence of several faults including the Hidden Spring and Powerline Faults, traces of which are mapped along a series of oases, seeps, and springs. Due to declining water levels in the Dos Palmas Preserve, several recharge basins have recently been installed 4 km northeast of the San Andreas Oasis, and while the water levels elsewhere in the Preserve have increased, the water levels at the Oasis are still in decline. Little is known about the subsurface structure in this area, making it difficult to determine the cause of declining water levels and an appropriate course of action. The Oasis is bound to west by the Hidden Spring Fault (HSF), a northwest-southeast trending fault that continues south of the Oasis for roughly 8 km as either a left step to the Powerline Fault (Clark, 1984) or as an uninterrupted fault (Babcock, 1969). Vegetation associated with the Oasis terminates abruptly along the trace of the HSF, indicating it is a barrier to groundwater flow.

To characterize the mapped and unmapped faults in this area, we used several geophysical techniques including ground-based magnetic, very low frequency (VLF), and direct current (resistivity) surveys. A GEM proton precession magnetometer with a VLF attachment was used to collect magnetic and VLF data. These measurements were used to create total magnetic intensity maps and profiles of Fraser filtered VLF data. High amplitude magnetic anomalies and Fraser tilt values correlate well with the mapped trace of the HSF, and may indicate the presence of a second, unmapped fault east of the Oasis. If the unmapped fault also prevents the flow of groundwater, similar to the HSF, these two faults could be funneling groundwater from the north. Resistivity profiles were collected with an IRIS Syscal Kid Switch unit with 24 electrodes, used at 2.5 and 5 m spacing, to generate resistivity models of the subsurface. Resistivity profiles traversing the HSF show a vertically oriented feature, which may represent saturated or clay rich fault gouge. Profiles collected further south, where there may be a left step in the HSF, show relatively low resistivity values, which may indicate the groundwater table is at or near the surface. To better understand faulting in this area and how it may affect the flow of groundwater, more magnetic, VLF, and DC resistivity data will be collected along the HSF and unmapped fault.

Citation
Petrashek, S. R., Pulver, N. W., Contreras, R., Faherty, D., & Polet, J. (2019, 08). Using Ground-based Magnetics, VLF, and DC Resistivity to Examine Faulting at the San Andreas Oasis, Dos Palmas Preserve. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)